Many hazardous environmental agents are harmful to human health because of their ability to damage the genome, which can lead to both inherited and somatic genetic disease. The pathologic effects of these agents on the genome include not only induction of mutations but also rearrangement, deletion and loss of chromosomes. Although most studies of genetic toxicity have focused on mutagenesis, recent research documents the major role that recombination and rearrangement of genomic DNA plays in both the correct repair of genotoxin-induced damage and pathologic responses that can result in disease. Although genetic recombination is an important component of the cellular response to genetic damage, we have only a limited understanding as to where, when and how it occurs in vivo. The long-range goal of this proposal is to develop a transgenic mouse model that can be used to further our knowledge of recombination in the living animal as well as assay environmental agents for their effects on the in vivo recombination of genomic DNA. This goal will be accomplished through the following specific aims: 1. To establish lines of transgenic mice which have integrated into their genomic DNA novel recombination vectors that contain tandem non- complementary mutants of a reporter gene( E coli lacZ). 2) To obtain baseline data regarding the frequency, rate and molecular nature of spontaneous intrachromosomal recombination between the lacZ genes contained in the integrated vector DNAs in the transgenic animals. Recombination will be detected in situ in the somatic and germ-line tissue of these animals through the use of assays that detect expression of E coli beta-galactosidase. Molecular analyses of LacZ+ putative recombinants will be carried out by Southern blotting and PCR. This work will focus on how recombination varies a) between tissues, b) during embryonic development, and c) with aging. 3) To study the effects of a model genotoxic agent, ionizing radiation, on the frequency and molecular nature of in vivo recombinational events in these animals. This project could lay the groundwork for comprehensive in vivo studies of mammalian recombination, create multiple opportunities for examining aspects of recombination that cannot be approached using current in vitro techniques and provide transgenic mice for novel assays of genotoxicity.